Let the first term be a and common difference be d
To Find: d
⇒ 2a + (n – 1)d = 2P + (n – 1)Q
⇒ 2(a – P) = (n – 1)(Q – d)
Put n = 1 to get the first term as sum of 1 term of an AP is the term itself.
⇒ P = a
⇒ (n – 1)(Q – d) = 0
For n not equal to 1 Q = d
Common difference is Q.